Structure and Function of Carbohydrate-Binding Module Families 13 and 42 of Glycoside Hydrolases, Comprising a beta-Trefoil Fold

Some carbohydrate-active enzymes display a modular structure in which catalytic modules that target an insoluble substrate are often attached to one or more noncatalytic carbohydrate-binding modules (CBMs) that assist enzymatic activity. CBMs have been classified into more than 60 families based on amino acid sequence similarities. CBM family 13 (CBM13) and family 42 (CBM42) possess a beta-trefoil fold and are grouped into CBM fold family 2. The beta-trefoil fold contains a sequence of approximately 45 amino acid residues that is repeated 3 times, resulting in three subdomains (alpha,beta and gamma) that fold into an overall globular structure. Each subdomain is composed of four beta-strands that fold into a Y-shaped beta-hairpin structure. CBM13 and CBM42 have multivalent sugar-binding ability. In this review, I describe the sugar-binding mechanisms of the CBM13 and CBM42 domains of a beta-xylanase, a beta-L-arabinopyranosidase, and an alpha-L-arabinofuranosidase.